1. Field of the Invention
This invention relates to a photo-plating solution and process which can form a thick metal or alloy coating containing at least one of copper, nickel, cobalt and tin on only the light-radiated portions of a semiconductor supported on a substrate, and which can be employed to form e.g., an electric-circuit pattern on a printed-circuit board.
2. Description of the Related Art
There is known a photo-plating process which utilizes radiant energy of light to deposit a metal selectively on the light-radiated portions of a substrate. More specifically, the photo-oxidation-reduction reaction of a semiconductor which has absorbed light is utilized to deposit a metal on only the light-radiated portions of a substrate on .which a semiconductor is supported, as disclosed in Japanese Laid-Open Patent Publication No. 205388/1990 and Japanese Patent Publication No. 2950/1990, and aqueous metal-ion solutions containing alcohol or form aldehyde as a reducing agent are used as plating solutions. Referring in more detail to the known process, a substrate on which a semiconductor, such as TiO.sub.2, is supported is immersed in an aqueous metal-ion solution containing a reducing agent, and ultraviolet light is applied to the semiconductor to form electrons on the conduction band thereof, so that the electrons may reduce the metal ion in the solution to deposit metal on the semiconductor. The reducing agent supplies electrons to holes formed in the valence band of the semiconductor by ultraviolet light, and prevents the holes from reacting with the electrons formed in the conduction band. The known process, however, has the drawback that if a metal layer having a thickness of several thousand angstroms is deposited on the semiconductor, it disables light to reach the semiconductor and effect the deposition of metal, resulting in the failure to form a thick metal layer.